1. Field of the Invention
The present invention relates to an optical waveguide device and a method of fabricating the same. More particularly, the invention relates to an optical waveguide device using at least one optical ring resonator having an expanded operable or usable frequency range to the upper frequency side, and a method of fabricating the device.
2. Description of the Related Art
In recent years, the transmission rate of optical fiber communication has been increasing continuously. For example, a transmission rate as high as 10 Gigabit per second (Gbit/s) has been actually introduced into commercial optical communication systems. Moreover, in the research and development fields of optical fiber communication, experimental results that 40 Gbit/s, 160 Gbit/s, and higher transmission rates were experimentally realized in optical fiber communication systems have been already reported so far.
In general, when the transmission rate is raised to 10 Gbit/s or higher, “wavelength dispersion” of an optical fiber will badly affect the performance of signal light transmission conspicuously, which often restricts the transmittable distance by way of optical fibers. This is because the velocity of signal light propagating through an optical fiber has wavelength dependence. Specifically, in general, spectral line broadening occurs in the oscillation mode due to the so-called wavelength chirping induced on optical modulation. If a transmission line or path has wavelength dispersion, the transmission rate of signal light propagating in the line or path will vary within the broadened spectrum line. Therefore, after long-distance transmission by way of optical fibers, the arriving time of the components of the signal light will vary according to their wavelength and as a result, the waveform of the modulated signal light will degrade or deform. Such the waveform degradation or deformation of the modulated signal light makes it difficult to reproduce its original waveform as desired.
To solve this problem, various types of “compensated optical fiber” have ever been extensively used. The compensated optical fiber includes a core with a diameter smaller than that of ordinary optical fibers, thereby generating negative wavelength dispersion therein. Due to the negative wavelength dispersion, wavelength dispersion occurring in an active optical fiber through which signal light is being transmitted is cancelled and as a result, possible waveform degradation of the propagating signal light is prevented.
However, the compensated optical fiber has many restrictions for use. For example, (i) the total length needs to be at least approximately 20 km, (ii) the input level of the signal light has to be sufficiently low in order to prevent the “four lightwave mixture” phenomenon due to optical non-linearity, and (iii) wavelength dispersion is unable to be adjusted at the setting-up scene or site for actually setting up optical fibers. Here, the “tour light-wave mixture” phenomenon is a phenomenon that a substance virtually absorbs two photons due to its non-linear polarization and then, emits two photons in such a way as to conserve energy. Additionally, the compensated optical fiber has a problem that the loss rate of the signal light is relatively high compared with ordinary optical fibers.
To solve the said problem about the loss rate and to eliminate the above-described restrictions of the compensated optical fiber, “waveguide-type dispersion compensators”, in particular those using the optical ring resonator, have ever been studied.
The use of the optical ring resonator as optical filters has been expected and researched for a long time. The waveguide-type optical ring resonator comprises a ring-shaped or circular optical waveguide (which may be simply referred as “waveguide” hereinafter) serving as a resonant, and an input/output optical waveguide for inputting signal light into the resonator waveguide and outputting signal light therefrom. The input/output waveguide is optically coupled with the resonator waveguide in the coupling section. A directional optical coupler is formed in the coupling section to optically couple these two waveguides together. By changing the refractive index of the directional coupler to thereby change the phase of the signal light, the branching ratio of the signal light with respect to the resonator waveguide is controllable. Thus, the finesse of the ring resonator itself is made controllable. Moreover, by changing the temperature of the circular resonator waveguide, the resonant wavelength is changeable. As a result, the ring resonator is operable as a wavelength-variable optical filter.
Furthermore, in recent years, researches have been conducted to positively utilize the wavelength dispersion characteristic of the ring resonator, thereby controlling the wavelength dispersion occurring in optical fibers. This is to cancel the degradation of the pulse waveform of signal light, which is induced by the wavelength dispersion characteristic of the optical fiber used, by generating opposed wavelength dispersion to that of the fiber, thereby suppressing the optical pulse-waveform degradation.
FIG. 1 shows an example of the structure of the prior-art optical dispersion compensation devices of this type. A first paper, ECOC 2000, Munich, Post-deadline paper, written by F. Horst, C. Berendsen, R. Beyeler, G. Bona, R. Germann, H. Salemink, and D. Wiesmann, entitled “Tunable ring resonator dispersion compensators realized in high-refractive-index contrast SiON technology”, discloses the optical dispersion compensation device 100 with an optical circuit formed by using the Planar Lightwave Circuit (PLC) technique.
The device 100 comprises a ring-shaped resonator waveguide 101 serving as an optical ring resonator, a linear input/output optical waveguide 102, and an optical directional coupler 103 for optically coupling the waveguides 101 and 102 to each other. The resonator waveguide 101 is made of an optical waveguide having a large refractive-index difference with respect to its surrounding material.
A plurality of the dispersion compensation devices 100 shown in FIG. 1 can be connected in cascade to each other to increase an obtainable amount of the wavelength dispersion. For example, when the four devices 100 were connected in cascade, the maximum wavelength dispersion was −3430 picosecond per nanometer (ps/nm) under the condition that the highest operable frequency was 12.5 GHz. This means that, if so, the wavelength dispersion of 200 km's worth of an ordinary optical fiber can be compensated. When the total length of the ring-shaped resonator waveguide 101 was set at approximately 4 mm, the highest operable frequency was prominently expanded to 25 GHz, in which the wavelength dispersion of 415 ps/nm was obtained
With the prior-art dispersion compensation device 100 of FIG. 1, a heater 104 is additionally provided to overlap with the ring-shaped resonator waveguide 101 and the straight input/output waveguide 102 in the directional coupler 103. By supplying electric power to the heater 104 to change the temperature of the corresponding pars of the waveguides 101 and 102, the finesse of the resonator waveguide 101 (i.e., the ring resonator) is controllable. If the finesse is raised, the wavelength dispersion is increased. Contrarily, if the finesse is lowered, the wavelength dispersion is decreased
The heater 104, which is located to overlap with the waveguide arms of the directional coupler 103, is provided for controlling or adjusting the wavelength of the signal light propagating in the resonator waveguide 101 by supplying electric power thereto. On the other hand, another heater 105 is provided to overlap with the remaining part of the ring-shaped waveguide 101 other than the coupler 103. The heater 105 is provided for controlling or adjusting the phase of the signal light propagating in the resonator waveguide 101 by supplying electric power thereto, thereby adjusting the finesse and wavelength dispersion.
FIG. 2 shows another example of the structure of the prior-art dispersion compensation devices. A second paper, OFC 2001, Anaheim, Calif., Post-deadline paper, PD9, written by C. K. Madsen et al., entitled “Compact Integrated Tunable Chromatic Dispersion Compensator with a 4000 ps/nm Tuning Range”, which corresponds to the U.S. Pat. No. 6,289,151 issued on Sep. 11, 2001, discloses a dispersion compensation device 120 with an optical circuit formed by using the PLC technique.
Unlike the device 100 shown in FIG. 1, the device 120 shown in FIG. 2 comprises a Mach-Zehnder optical interferometer 122 introduced into an optical ring resonator 121. The interferometer 122 has two optical waveguide arms 121A and 121B, which are equal in length to each other. Thus, the interferometer 122 has a symmetrical structure with respect to its central line. The arms 121A and 121B are optically coupled to each other by way of optical directional couplers 123A and 123B. Each of the couplers 123A and 123B constitutes a 2×2 (i.e. two input and two output) optical coupler. The arms 121A and 121B are intersected to each other and then, connected to two input/output optical waveguides 124A and 124B by way of the directional couplers 123A and 123B, respectively.
With the prior-art dispersion compensation device 120 of FIG. 2, signal light propagating through one of the input/output waveguides 124A and 124B flows into the ring resonator 121 by way of the coupler 123A or 123B at a predetermined branching ratio and then, flows out of the resonator 121 by way of the coupler 123B or 123A at a predetermined branching ratio to the other of the input/output waveguides 124A and 124B.
A heater 126, which is provided to overlap with the ring resonator 121, is used for wavelength adjustment. A heater 127, which is provided to overlap with the arm 121B in the interferometer 122, is used for phase adjustment.
The finesse of the ring resonator 121 is controlled by refractive index difference between the arms 121A and 121B and/or the use of the heater 127, which resulted in the wavelength dispersion of ±1980 ps/nm. The highest operable frequency was 13.4 GHz. In the second paper, a signal transmission experiment at 10 Gb/s was carried out using the dispersion compensation device 120 of FIG. 2 and a Non Return to Zero (NRZ) signal and as a result, a fact that desired dispersion compensation characteristic and desired transmission characteristic were realizable was confirmed.
As explained above, it is understood that an optical circuit for compensating the wavelength dispersion of the optical fiber is realizable with an optical ring resonator using the PLC technique. However, with the conventional dispersion compensation devices (including the above-described prior-art dispersion compensation devices 100 and 120) using an optical ring resonator that have been reported so far, the highest operable frequency is as low as approximately 25 GHz. To shift the highest operable frequency toward the shorter wavelength (i.e., higher frequency) side, the total length of the optical ring resonator itself needs to be as short as possible.
However, with the above-described prior-art dispersion compensation devices 100 and 120, as shown in FIGS. 1 and 2, an optical coupler such as the directional coupler 103 or Mach-Zehnder interferometer 122 is provided. Since the said optical coupler necessitates a comparatively large size, a limit exists in reducing the overall waveguide length of the ring resonator 101 or 121. This limit restricts the highest operable frequency of the devices 100 and 120 to a level of approximately 25 GHz.